1. Field of the Invention
The invention relates to a molecular fluorine (F2) laser, and particularly to an F2-laser with an improved resonator design and improved beam monitoring and line-selection for providing stable output beam parameters at high operating repetition rates.
2. Discussion of the Related Art
a. VUV microlithography
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, as well as the following generation of ArF-excimer laser systems operating around 193 nm. Vacuum UV (VUV) will use the F2-laser operating around 157 nm.
The construction and electrical excitation of the F2-laser differs fundamentally from the rare gas-halide excimer lasers mentioned above. The laser gas of a rare gas-halide excimer laser, such as the KrF or ArF laser, includes a laser active molecular species that has no bound ground state, or at most a weakly bound ground state. The laser active gas molecule of the excimer laser dissociates into its constituent atomic components upon optical transition from an upper metastable state to a lower energy state. In contrast, the laser active gas constituent molecule (F2) of the F2-laser responsible for the emission around 157 nm is bound and stable in the ground state. In this case, the F2 molecule does not dissociate after making its optical transition from the upper to the lower state.
The F2-laser has an advantageous output emission spectrum including one or more lines around 157 nm. This short wavelength is advantageous for photolithography applications because the critical dimension (CD), which represents the smallest resolvable feature size producible using photolithography, is proportional to the wavelength. This permits smaller and faster microprocessors and larger capacity DRAMs in a smaller package. The high photon energy (i.e., 7.9 eV) is also readily absorbed in high band gap materials like quartz, synthetic quartz (SiO2), Teflon (PTFE), and silicone, among others, such that the F2-laser has great potential in a wide variety of materials processing applications. It is desired to have an efficient F2 laser for these and other industrial, commercial and scientific applications.
b. line-selection and line-narrowing
The emission of the F2-laser includes at least two characteristic lines around xcex1=157.629 nm and xcex2=157.523 nm. Each line has a natural linewidth of less than 15 pm (0.015 nm), and in the usual pressure range between 2-4 bar, the natural linewidth can be less than 2 pm. The intensity ratio between the two lines is I(xcex1)/I(xcex2)0.7. See V. N. Ishenko, S. A. Kochubel, and A. M. Razher, Sov. Journ. QE-16, 5 (1986). FIGS. 1a and 1b illustrate the two above-described closely-spaced peaks of the F2-laser spontaneous emission spectrum. FIG. 1b shows a third F2 laser emission line around 157 nm that is observed when neon is used as a buffer gas, but that is not observed when the buffer gas used is strictly helium, as shown in FIG. 1a (see U.S. patent application Ser. No. 09/317,526, which is assigned to the same assignee as the present application and is hereby incorporated by reference). Either way, the characteristic bandwidth of the 157 nm emission of the F2 laser is effectively more than 100 pm due to the existence of the multiple lines.
Integrated circuit device technology has entered the sub-0.18 micron regime, thus necessitating very fine photolithographic techniques. Line narrowing and tuning is required in KrF- and ArF-excimer laser systems due to the breadth of their natural emission spectra (around 400 pm). Narrowing of the linewidth is achieved most commonly through the use of a line-narrowing unit consisting of one or more prisms and a diffraction grating known as a xe2x80x9cLittrow configurationxe2x80x9d. However, for an F2-laser operating at a wavelength of approximately 157 nm, use of a reflective diffraction grating is unsatisfactory because a typical reflective grating exhibits low reflectivity and a laser employing such a grating has a high oscillation threshold at this wavelength. The selection of a single line of the F2 laser output emission around 157 nm has been advantageously achieved and described at U.S. patent application Ser. Nos. 09/317,695 and 09/317,527, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference. It is desired to improve upon the line-selection techniques set forth in the ""695 and ""527 applications. Moreover, it is desired to have a way of monitoring the quality of the line selection being performed.
For an excimer laser, such as a KrF- or ArF-excimer laser, the characteristic emission spectrum may be as broad as 400 pm. To narrow the output bandwidth, one or more dispersive line-narrowing optics are inserted into the resonator. To increase the angular (and spectral) resolution commonly more than one optical dispersive element is introduced. A typical line-narrowing arrangement for a KrF- or ArF-excimer laser includes a multiple prism beam expander before a grating in Littrow configuration. The length of the resonator increases as more optical elements as added. Also, additional optical interfaces gives rise to losses which result in a decrease of the output power for a given input voltage. It is desired to reduce the number of optical components in a line-narrowed excimer laser.
c. absorption
The F2-laser has been known since around 1977 [see, e.g., Rice et al., VUV Emissions from Mixtures of F2 and the Noble Gases-A Molecular F2 laser at 1575 angstroms, Applied Physics Letters, Vol. 31, No. 1, Jul. 1, 1977]. However, previous F2-lasers have been known to exhibit relatively low gains and short gas lifetimes. Other parameters such as the pulse-to-pulse stabilities and laser tube lifetimes have been unsatisfactory. In addition, oxygen and water exhibit high absorption cross sections around the desired 157 nm emission line of the F2-laser, further reducing overall efficiency at the wafer when encountered by the laser beam anywhere along its path. To prevent this absorption, one can maintain a purged or evacuated beam path for the F2-laser free of oxygen, hydrocarbons and water (see U.S. patent application Ser. No. 09/343,333, which is assigned to the same assignee as the present application and is hereby incorporated by reference). In short, despite the desirability of using short emission wavelengths for photolithography, F2-lasers have seen very little practical industrial application to date. It is desired to have an F2-laser with enhanced gain, longer pulse lengths, enhanced energy stability, and increased lifetime.
F2-lasers are also characterized by relatively high intracavity losses, due to absorption and scattering in gases and optical elements within the laser resonator, particularly again in oxygen and water vapor which absorb strongly around 157 nm. The short wavelength (157 nm) is responsible for the high absorption and scattering losses of the F2-laser, whereas the KrF-excimer laser operating at 248 nm does not experience losses of such a comparably high degree. In addition, output beam characteristics are more sensitive to temperature induced variations effecting the production of smaller structures lithographically at 157 nm, than those for longer wavelength lithography such as at 248 nm and 193 nm. Therefore, the advisability of taking steps to optimize resonator efficiency is recognized in the present invention.
d. atomic fluorine visible emission
The VUV laser radiation around 157 nm of the F2-molecule has been observed as being accompanied by further laser radiation output in the red region of the visible spectrum, i.e., from 630-780 nm. This visible light originates from the excited fluorine atom (atomic transition). It is desired to have an F2-laser wherein the output in the visible region is minimized and also to maximize the energy in the VUV region.
Although the active constituent in the gas mixture of the F2-laser is fluorine, the amount of pure fluorine amounts to no more than about 5 to 10 mbar of partial pressure within the gas mixture, and typically less than 5 mbar. A higher overall pressure is needed to sustain a uniform discharge. Consequently, a buffer gas is needed to raise the discharge vessel pressure, typically to well above atmospheric pressure (e.g., 2-10 bars), in order to achieve an efficient excitation within the discharge and realize an efficient output of the 157 nm radiation.
For this reason, F2-lasers have gas mixtures including an inert buffer gas which is typically helium and/or neon. When helium is used, however, the output in the red visible region can range from one to more than three percent of the VUV emission. In addition, the VUV pulse lengths tend to be relatively short. The visible output of the F2 laser has been advantageously reduced by using neon or a combination of neon and helium as the buffer gas in the F2 laser (see the ""526 application). In addition, the length of the VUV output pulses of the F2 laser has been shown in the ""526 application to be increased using neon in the gas mixture improving both line selection and line narrowing capability. It is desired to further reduce the influence of the visible emission on the performance of the F2 laser.
e. relatively short pulse duration
As noted above, the pulse duration of the F2 laser is relatively short compared with that of the rare gas-halide excimer lasers. For example, KrF laser pulses make between four and six roundtrips through the laser resonator, whereas F2 laser pulses typically make only one to two roundtrips. This reduces the effectiveness of the line-selection and narrowing components of the resonator. The short pulse duration also reduces the polarizing influence of surfaces aligned at Brewster""s angle such as the windows on the laser tube or of other polarizing components in the resonator. The pulse duration is advantageously increased as described in the ""526 application using neon in the gas mixture. A comparison of the F2 laser emission linewidths in FIG. 1a with those shown at FIG. 1b illustrate the effect of increasing the pulse duration by substituting neon for helium in the gas mixture. However, when the laser tube windows are aligned at Brewster""s angle, the output laser beam is still only about 70% polarized. It is desired to have a F2 laser which emits a substantially polarized beam, e.g., such that the beam exhibits a 98% or greater polarization.
f. beam parameter and alignment monitoring
It is desired that the pulse energy, wavelength and bandwidth of the output beam each be stabilized at specified values, particularly for photolithography lasers. Moreover, it is desired to maintain a substantially constant energy dose at the workpiece. Further, it is desired to maintain a steady and predetermined beam alignment. Various techniques are known for monitoring the pulse energy and/or other beam parameters and controlling the discharge voltage, the composition of gases in the laser tube and/or the hardware and optics for stabilizing these parameters in photolithography lasers (see U.S. patent application Ser. Nos. 09/447,882, 60/124,785, 60/171,717, 09/418,052, 60/159,525, 09/416,344, 09/379,034, 60/127,062, 60/160,126 and 09/513,025, each of which is assigned to the same assignee as the present application and is hereby incorporated by reference. Beam alignment techniques are described at U.S. Pat. No. 6,014,206 and U.S. patent application Ser. No. 09/271,020, which are assigned to the same assignee as the present application, and U.S. Pat. No. 5,373,515, each of which is hereby incorporated by reference. The visible emission of the F2 laser and the tendency of the VUV emission of the F2 laser to undergo absorption present some difficulties when applying the techniques particularly set forth in these patents and patent applications. It is therefore desired to effectively implement beam alignment and parameter monitoring techniques in a F2 laser system.
It is an object of the invention to provide an efficient F2 laser for industrial, commercial and scientific applications such as photolithography and other materials processing applications.
It is also an object of the invention to provide improved line-selection techniques for the F2 laser, and to provide a technique for monitoring the quality of the line selection being performed.
It is a further object of the invention to provide a F2 laser having improved resonator efficiency.
It is another object of the invention to provide techniques for reducing the influence of the visible emission on the performance of the F2 laser.
It is also an object of the invention to provide a F2 laser that emits a substantially polarized beam, e.g., such that the beam exhibits a 98% or greater polarization.
It is a further object of the invention to effectively implement beam alignment and parameter monitoring techniques in a F2 laser system.
It is an additional object of the invention to provide an excimer laser, such as a KrF or ArF excimer laser having an efficient line-narrowing optical resonator.
In accordance with the above objects, a F2 laser in accord with the present invention includes a laser tube filled with a laser gas mixture and having a plurality of electrodes connected with a power supply circuit for energizing the gas mixture. A laser resonator including the tube for generating a 157 nm laser beam includes a line selection unit for selecting one of multiple closely-spaced characteristic emission lines around 157 nm.
In a first aspect of the invention, line selection is provided by a transmission diffraction grating. The preferred grating is made of CaF2 and also serves to outcouple the laser beam. The transmission grating in accord with the first aspect advantageously permits a straight, shortened and more efficient F2 laser resonator.
In a second aspect of the invention, line selection is provided for a F2 laser by a grism. The preferred grism also serves either to outcouple the beam or as a highly reflective resonator reflector. The grism in accord with the second aspect advantageously provides enhanced dispersion and efficiency.
Also in accord with this aspect of the invention, an excimer or molecular fluorine laser includes a laser output coupler including a grism, which is a combination of a prism and a grating. Such a combination of prism and grating within one element advantageously improves the resolving power of a single dispersive element and reduces the internal resonator losses by a minimum of optical interfaces.
The grism used directly as an output coupler for an excimer or molecular fluorine laser advantageously combines four different tasks in one element: partial reflection (or output coupling) for the resonator, dispersion and line narrowing or line selection, suppression of background radiation, such as amplified spontaneous emission (ASE) radiation (or a parasitic second line), and pointing stabilization of the selected line (wavelength). The grating-prism (or grism) may be designed in such a manner that it realizes a straight beam path for the selected wavelength which is used as the output beam for use with an application process, operating like a common mirror if the blaze angle of the grating is equal to the prism angle for the selected wavelength. Thus, the laser resonator can be very short even if it is containing a line selecting or line narrowing element having the direction of beam propagation in a straight line.
In a third aspect of the invention, line selection for an F2 laser is fully performed at the front optics module of the laser resonator resulting in a more efficient resonator.
In a fourth aspect of the invention, a monitor grating and an array detector are provided for monitoring and controlling the intensity of the selected (and/or unselected) lines and for monitoring the stability of the selected wavelength. The quality of the line selection may be advantageously monitored.
In a fifth aspect of the invention, a F2 laser system includes an energy detector provided in an enclosure purged with an inert gas at a slight, regulated overpressure. Advantages include reduced turbulence typically associated with high gas flow and a reduced rate of deposition of contaminants on optical surfaces. The fifth aspect may be advantageously combined with the fourth aspect.
In a sixth aspect of the invention, a blue or green reference beam (e.g., having a wavelength between 400 nm and 600 nm) is used for F2 laser beam wavelength calibration and/or alignment stabilization. The blue or green reference beam advantageously is not reflected out with the red emission of the laser and is easily resolved from the red emission.
In a seventh aspect of the invention, the clearing ratio of the laser gas flow through the discharge area of a F2 laser is improved. The reduced clearing time is provided by narrowing the discharge width using improved laser electrodes and/or by increasing the gas flow rate through the discharge while maintaining uniformity by using a more aerodynamic discharge chamber. F2 laser operation at higher repetition rates is advantageously permitted by the reduced clearing ratio in accord with the seventh aspect of the invention.
In an eighth aspect of the invention, a F2 laser is provided with a substantially polarized output beam. The polarization is provided by a thin film polarizer, a double reflection prism and/or Brewster windows. The polarization provided by the eighth aspect of the invention is advantageously 98% or better.